The present invention relates to a ballast circuit for a fluorescent lamp, and more particularly, to a ballast circuit for starting either a fluorescent lamp (FL) or a compact fluorescent lamp (CFL).
FIG. 1 schematically illustrates a ballast circuit for a fluorescent lamp recited in U.S. Pat. No. 4,647,817. In FIG. 1, the lamp shown is 15W CFL. The operating frequency applied to the lamp of the ballast circuit is 45 KHz. A 220V, 50 Hz or 110V 60 Hz power supply is connected to terminals 2 and 3 to power the lamp 1. The input voltage U.sub.N is connected to a filter 4. Then, the filtered alternating voltage is provided to a rectifier 5 for rectification. The rectified voltage is smoothed by a capacitor 6. The filtered and smoothed voltage is applied to an inverter INV which comprises two transistors 7 and 8 having emitter resistors 9 and 10, respectively, and an inverter controlling circuit 11. The inverter INV is the essential element in this operating device.
The controlling voltage for the inverter is obtained from a transformer 12 having a primary coil 13 of only several windings. The primary coil winding 13 is connected to an operating circuit of the lamp 1.
The inverter INV basically generates a rectangular wave voltage, which is applied to the lamp 1 through an inductor 14 and a blocking capacitor 15 in the operating circuit. The blocking capacitor 15 blocks direct current (DC) from the lamp and forms a resonance circuit portion with the inductor 14. For operation at 45 KHz, the inductor 14 has an inductance of about 3 mH and the blocking capacitor 15 has a capacitance of about 47 nF.
The ignition and starting circuit ST is connected in parallel to the lamp 1 and serially with the electrodes 16 and 17 thereof, which comprises a parallel circuit of a limiting capacitor 19 and a positive temperature coefficient (PTC) resistor 20 and a starting capacitor 18. In the above circuit, the capacitance of the starting capacitor 18 is about 3.3 nF. A series circuit of the capacitors 18 and 19 form a coupled resonance capacitor C.sub.R. A resistor, C890, manufactured by Siemens is used as the PCT resistor 20.
FIG. 2 is a ballast circuit for a fluorescent lamp recited in U.S. Pat. No. 5,223,767. In FIG. 2, a pair of terminals 21 and 22 of an EMI filter 25 comprising a first capacitor 9 and a first inductor 26, are supplied with a low frequency AC power source, i.e., 120V, 60 Hz. The output of the EMI filter 25 is connected to terminals 23 and 24 of a voltage multiplier circuit 28 through a second inductor 27 and a second capacitor 10. The first and second inductors 26 and 27 are connected in series between the AC input terminal 22 and an input terminal 24 of the voltage multiplier circuit 28. The first capacitor 9 is combined to both ends of the input terminals 21 and 22 and the second capacitor 10 is connected between the terminal 1 and a node of the inductors 26 and 27. The voltage multiplier circuit 28 having a pair of diodes 31 and 32 are connected in series to both ends of DC input terminals 33 and 34 of a high frequency DC/AC half bridge inverter 35. Buffer capacitors 36 and 37 connected in series are combined parallel to the diodes connected in series. A pair of switching transistors 38 and 39 are connected in series and arc combined to the DC power source terminals 33 and 14. Third and fourth diodes 57 and 58 arc combined to the far ends of the transistors 38 and 39, respectively. One end of a discharge lamp 40 (for example, the fluorescent lamp) is combined to a connection point 23 between the diodes 31 and 32 through a capacitor 41 and the other end of the discharge lamp 40 is connected to a connection point 42 between the switching transistors 38 and 39 through an inductor 43. This connection is indicated as a dashed line in the drawing. However, according to the present invention, in the circuit constituted of the first arid second inductors 44 and 45 instead of the single inductor 43, the common node between the first and second inductors 44 and 45 is connected to the DC input terminal 34 through a capacitor 46.
Electrodes of the discharge lamp 47 and 48 are connected parallel to a PTC resistor 49 and a capacitor 50 which are connected in series. The PTC resistor 49 provides a path through which a pre-heating current for heating the electrodes of the lamp flows before igniting the lamp. A buffer capacitor 52 operates so as to connect the node 42 to a node 53 to reduce loss of power in the switching transistors 38 and 39.
A conventional control circuit 54 operates the switching transistors 38 and 39 in an alternating ON/OFF manner in which when one transistor is turned on, the other transistor is turned off. The control circuit, driven by an IC, however, can be constituted by a transformer having first and second windings, respectively combined to the windings connected in series such as the inductors 44 and 45, the base and the emitter of the switching transistor, as a load circuit connected between the node 42 and the node 53. A magnetic oscillation high frequency DC/AC converter can be obtained since the second winding can be connected in series to the base of the transistor in this way in the respective resistors. Since the exact driving method of the switching transistor is not limited to this method and various driving methods can be used, a desirable operation of the circuit is performed.
The lamp 40, the capacitor 51, the inductors 44 and 45, and the capacitor 46 in essence constitute a resonant circuit which makes the half bridge inverter perform oscillation at high frequency.
A starting circuit 55 is provided to start the operation of a high frequency inverter 35.
The input current from the AC power source provides a high frequency path for supplying resonance power to a capacitor 41 and the lamp circuit and returns the same to the electrolytic buffer capacitors 36 and 37. Therefore, the capacitor 30 and the inductor 27 which make the voltages of the respective ends of the two capacitors higher than the voltage between the lines of the AC power source terminals 21 and 22 are part of the multiplier voltage circuit 28. The energy which returns from the resonance circuit through the capacitor 41 brings about the voltage generation of both ends of the inductor 27. This voltage is added to the voltage between the AC lines and generates a voltage which is higher than the buffer capacitor voltage, however, is clamped to the buffer capacitor voltage by the respective diodes 31 and 32. Therefore, power returns to the buffer capacitor through the diodes 31 and 32. The additional voltage boosting operation of the buffer capacitors 36 and 31 is provided by an LC resonant circuit comprised of the inductor 45 arid the capacitor 46. By using the LC resonant circuit instead of a single inductor such as the inductor 43, the feedback voltage supplied to the capacitors 36 and 37, namely, a partial voltage derived from the capacitor 41 and another partial voltage derived from the LC resonant circuits 44, 45, and 46, is effectively divided.
The diodes 57 and 58 provide a path through which power is returned to the buffer capacitor. In the case that the transformer is used in driving the transistors 38 and 39, the diodes 57 and 58 can be removed since the second winding and the collector-base connection of the transistors provide a low impedance path for returning the energy to the capacitor.
The collector-base connection of the transistor provides the function of the diodes. Furthermore, the boosting operation is provided by the inductor 27 which operates as a voltage source which moves electrical currents to the capacitors 36 and 37 through the classification diodes 31 and 32.
In another embodiment of the circuit, a capacitor 56 can be connected between the node 53 and the input terminal 54. The half bridge inverter circuit will operate like the high frequency boosting converter which boosts the respective voltages of the buffer capacitors 36 and 37 to more than a peak line voltage by an appropriate selection of the capacitors 41 and 56, thereby providing the circuit with a high power factor and low harmonic line current. Thus, the circuit draws much less current due to the improved power factor.
As described above, since the input ripple currents to the capacitors 36 and 37 are not totally derived from the AC power source of low frequency (60 Hz) and are partially derived from the half bridge circuit of high frequency, lower capacitors can be used, and moreover, can maintain a low ripple voltage.
However, in the ballast circuit of FIG. 1, the waveform the capacitors 36 and 37 are not totally derived from the AC power source of low frequency (60 Hz) and are partially derived from the half bridge circuit of high frequency, smaller capacitors can be used, and moreover, can maintain a low ripple voltage.
However, in the ballast circuit of FIG. 1, the waveform of the power source voltage output from the circuit, as shown in FIG. 3(c) compared to the power source voltage, shown in FIG. 3(a) is distorted by the current flows shown in FIG. 3(b). That is, the total harmonic distortion (THD) is about 140.about.370%, thereby generating a current wave form having a high current harmonic noise and generating much distortion in the waveform of the output voltage. This causes the change in a reference power source voltage. Also, the power factor is low, i.e., approximately 50.about.60%.
Also, the ballast circuit for the fluorescent lamp shown in FIG. 2 can be used with a power source of about AC 110V by adopting a voltage multiplier to the input power source. However, in the case that a power source voltage of AC 220V.about.230V is input, since 220V.times.2.sqroot.2(=616V) is output when performing double voltage amplification, the circuit device cost gets higher, namely, an electrolytic capacitor and a power transistor which can withstand high voltages or FET must be used. Also, the power factor is deteriorated by the inductor 27 which is used for boosting.